A proposed configuration of the power supply system includes a battery that is connected with a battery voltage-system power line via a system main relay, a first capacitor mounted to a first power line, an inverter configured to convert a DC power of a second power line to a three-phase AC power and supply the three-phase AC power to a motor, a second capacitor mounted to the second power line, and a step-up/down converter connected with both the first power line and the second power line (for example, JP 2016-129460A). The power supply system of this configuration uses a precharge circuit included in a system main relay to pre-charge the first capacitor and the second capacitor at a system start time, and, in the case of an abnormality of power consumption during pre-charging, diagnoses the place of an abnormality based on a battery voltage, a voltage of the first capacitor, a voltage of the second capacitor and a voltage of an auxiliary machine.